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Format datetime from string "20151210T11:25:11123"

I have string format "20151210T11:25:11123", can't convert to type DateTime in C# help me?
string date = "20151210T11:25:11123";
DateTime datea = DateTime.ParseExact(date, "dd/MM/yyyy hh:mm tt", CultureInfo.InvariantCulture);

You are using a time of 20151210T11:25:11123 but telling it to parse it as if it were formatted as dd/MM/yyyy hh:mm tt. The format does not match the string, so you get a FormatException. You need to provide a format that matches the string you have. It isn't clear to me what the last 5 digits are but a format like yyyyMMddThh:mm:ssfff will parse the string as 12/10/2015 11:25:11 AM. You may need to adjust the last part of the format to match whatever is actually encoded there in your string.
string date = "20151210T11:25:11123";
DateTime datea = DateTime.ParseExact(date, "yyyyMMddThh:mm:ssfff", CultureInfo.InvariantCulture)
Console.WriteLine(datea); // 12/10/2015 11:25:11 AM

with value datetime string ="20160121T13:26:24090"
code error :
DateTime datea = DateTime.ParseExact(date, "yyyyMMddThh:mm:ssfff", CultureInfo.InvariantCulture)
Console.WriteLine(datea); // 12/10/2015 11:25:11 AM

DateTime convert in c#

I have DateTime variable in this format "dd/mm/yyyy hh:mm:ss".
I would like to convert this to "mm/dd/yyyy hh:mm:ss". (Month before day)
I tried to use DateTime.parse like this:
DateTime dt = new DateTime();
dt = dateTimePicker1.Value;
dt = DateTime.Parse(dt.ToString(), "mm/dd/yyyy", null);
but it wont work.
I also try it with DateTime.ParseExact() but still nothing happened.
Any suggestion?

A DateTime doesn't have any implicit format. It just has date and time values.
Formatting subject only applies when you try to get it's textual representation. If you wanna get string represetation of it, you can use .ToString() method like;
string s = dateTimePicker1.Value.ToString("MM/dd/yyyy hh:mm:ss",
CultureInfo.InvariantCulture);
And remember, mm specifier is for minutes, MM specifier is for months. Also hh specifier is for 12-hour clock and HH specifier is for 24-hour clock representations. You might wanna use HH instead.
I used InvariantCulture as a second parameter because / and : have special meaning as replace me current culture or supplied culture date or time separator.

string date = dateTimePicker1.Value.ToString("MM/dd/yyyy");

You can also use this below code for advance:
GregorianCalendar cal = new GregorianCalendar(); // or any type of calendar you want ...
string formatted = string.Format("{1:00}/{2:00}/{0} {3:00}:{4:00}:{5:00} ",
cal.GetYear(dt), cal.GetMonth(dt), cal.GetDayOfMonth(dt) , cal.GetHour(dt), cal.GetMinute(dt), cal.GetSecond(dt));

DateTime time = DateTime.Now;
string format = "MMM ddd d HH:mm yyyy";
Console.WriteLine(time.ToString(format));

String to date conversion in c# as yyyy/mm/dd

I would like convert string content to date format as yyyy/MM/dd HH:mm:ss tt
string date = "2014-11-20 3:21:00 PM";
DateTime date_=System.DateTime.Now;
var result = DateTime.TryParseExact(date, "yyyy-MM-dd HH:mm:ss tt",
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None,
out date_);
But it returns result doesn't match requirement also TryParse function return false. If time zone is not defined, it will return expected result.

HH specifier is for 24 hour clock which is 00 to 23.
You need to use h specifier instead which represents 1 to 12 in 12 hour clock.
Also you don't need to initialize your out parameter value. Definition will be enough like;
string date = "2014-11-20 3:21:00 PM";
DateTime date_;
var result = DateTime.TryParseExact(date, "yyyy-MM-dd h:mm:ss tt",
CultureInfo.InvariantCulture,
DateTimeStyles.None, out date_);

The DateTime data type format is always MM/dd/yyyy hh:mm:ss tt (i.e. Date = {11/20/2014 12:00:00 AM}),
If you want to display the value you can change the format by using ToString extention method

Converting string to date time

I have a date which comes in a string like so:
09/25/2014 09:18:24
I need it like this (yyyy-mm-dd):
2014-09-25 09:18:24
The object that this date goes into is a nullable date.
Tried this does not work:
DateTime formattedDate;
bool result = DateTime.TryParseExact(modifiedDate, "yyyy-MM-dd",
CultureInfo.InvariantCulture,
DateTimeStyles.None,
out formattedDate);
Any clues?
Thanks in advance.

From DateTime.TryParseExact
Converts the specified string representation of a date and time to its
DateTime equivalent. The format of the string representation must
match a specified format exactly.
In your case, they are not. Use yyyy-MM-dd HH:mm:ss format instead.
string s = "2014-09-25 09:18:24";
DateTime dt;
if(DateTime.TryParseExact(s, "yyyy-MM-dd HH:mm:ss",
CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt))
{
Console.WriteLine(dt);
}
It is a little bit unclear but if your string is 09/25/2014 09:18:24, then you can use MM/dd/yyyy HH:mm:ss format instead. Just a tip, "/" custom format specifier has a special meaning as replace me with current culture or supplied culture date separator. That means, if your CurrentCulture or supplied culture's DateSeparator is not /, your parsing operation will fail even if your format and string matches exactly.
If you have already a DateTime and you want to format it, you can use DateTime.ToString(string) method like;
dt.ToString("yyyy-mm-dd", CultureInfo.InvariantCulture); // 2014-09-25
or
dt.ToString("yyyy-mm-dd HH:mm:ss", CultureInfo.InvariantCulture); // 2014-09-25 09:18:24
Remember, a DateTime does not have any implicit format. It just contains date and time values. String representations of them have formats.

In answer to your question, to convert it as you prefer, do it like this:
string originalDate = "09/25/2014 09:18:24";
DateTime formattedDate;
if (DateTime.TryParseExact(originalDate, "MM/dd/yyyy HH:mm:ss", CultureInfo.InvariantCulture, DateTimeStyles.None, out formattedDate))
{
string output = formattedDate.ToString("yyyy-mm-dd HH:mm:ss", CultureInfo.InvariantCulture);
}
And then output will have your desired format.

DateTime dateOf = Convert.ToDateTime("09/25/2014 09:18:24");
string myFormat = "yyyy-mm-dd";
string myDate = dateOf.ToString(myFormat); // output 2014-18-25
Datetime format

How to find exact format of a date string?

I need to know exact date format that will perse the string 16-Aug-78 12:00:00 AM. I have tried various string like "ss-MMM-yy hh:mm:ss". Is there any way for finding it to converting to any general format. I am using CultureInfo.InvariantCulture class.

Your string format is wrong. It has to match your string format exactly. You can use dd-MMM-yy hh:mm:ss tt format instead.
Here an example in LINQPad.
string s = "16-Aug-78 12:00:00 AM";
var date = DateTime.ParseExact(s, "dd-MMM-yy hh:mm:ss tt",
CultureInfo.InvariantCulture);
date.Dump();
Custom Date and Time Format Strings
Or, since dd-MMM-yy hh:mm:ss tt is a standart date and time format for InvariantCulture, you can directly DateTime.Parse method like;
string s = "16-Aug-78 12:00:00 AM";
var date = DateTime.Parse(s, CultureInfo.InvariantCulture);
date.Dump();
Here a demonstration.
Is there any way for finding it to converting to any general format?
There is no way to get format of a string except you create your own formatting. Only you can know what is your string format exactly, computer can't.
For example; 01/02/2014 can be 1 February 2014 or 2 January 2014 depends on which custom format you can parse it.

Try as below
var dstring = "16-Aug-78 12:00:00 AM";
DateTime result;
var matchingCulture = CultureInfo.GetCultures(CultureTypes.AllCultures).FirstOrDefault(ci => DateTime.TryParse(dstring, ci, DateTimeStyles.None, out result))

You have wrong format string, you are using ss for day it should be dd. This article Custom Date and Time Format Strings explains what you need for custom format.
Use
"dd-MMM-yy hh:mm:ss tt"
Instead of
"ss-MMM-yy hh:mm:ss"
You can use DateTime.ParseExact to parse the string.
DateTime dt = DateTime.ParseExact("16-Aug-78 12:00:00 AM", "dd-MMM-yy hh:mm:ss tt", CultureInfo.InvariantCulture, DateTimeStyles.None);

You can try with DateTime.TryParseExact():
string strDate = "16-Aug-78 12:00:00 AM";
DateTime datDate;
if(DateTime.TryParseExact(strDate , new string[] {"dd-MMM-yy hh:mm:ss tt" },
System.Globalization.CultureInfo.InvariantCulture,
System.Globalization.DateTimeStyles.None, out datDate))
{
Console.WriteLine(datDate);
}
else
{
Console.WriteLine("Error in datetime format");
}